In the field of lean-burn internal combustion engines, reduction of NOx, such as NO and NO2, in exhaust gas is a well-known problem. It is well known to use a Lean NOx Trap (LNT) in the exhaust system for adsorbing NOx when the engine is run lean and when the amount of NOx stored in the LNT reaches a predefined level to convert the stored NOx into N2 (nitrogen gas) during a NOx purge regeneration process in which the engine is run rich.
The term ‘lean’ as meant herein means an air-fuel ratio (lambda) above stoichiometry (where lambda at stoichiometry is equal to 1), that is to say, above the stoichiometric air-fuel ratio, where the production of HC (hydrocarbons) and CO (carbon oxides) are low and where the production of NOx is high. The term ‘rich’ as meant herein means an air-fuel ratio value with a lambda below 1 where the production of HC and CO used as reductants in the regeneration process is high and where the production of NOx and level of 02 are relatively low.
After completion of the NOx purge regeneration process the engine is run lean again and NOx is again adsorbed in the LNT.
The optimum NOx conversion temperature for a LNT NOx purge is dependent on several factors, such as the composition of the fuel used, the LNT construction in terms of the materials used, and the age of the LNT. However, in general terms, the optimum NOx conversion temperature value lies in a temperature region where optimum conversion of NOx into N2 is possible.
The temperature of exhaust gas supplied to a LNT varies and is generally increased when the rotational speed of the engine increases, when the load on the engine is increased, and in particular when the engine is run rich.
The inventors herein have recognized that one problem with known LNT NOx purge methods occurs when the engine is being run in a condition where the temperature of the exhaust gas flowing to the LNT is well below the optimum NOx conversion temperature, such as can often occur during light duty running such as urban or city driving.
If the temperature of the LNT is below an optimum temperature range of circa 300 to 400° C. and the NOx purge regeneration process starts by running the engine rich, then NOx will be released or purged from the LNT but, because the catalyst materials contained within the LNT are not active at such a low temperatures, the released NOx cannot be converted and will result in a sudden large increase in the NOx emissions from the tailpipe. It is therefore desirable to increase the temperature of the exhaust gas flowing to the LNT before starting the NOx purge regeneration process if the temperature of the LNT is below the optimum range if a sudden increase in NOx emissions is to be avoided.
Another factor affecting the performance of an LNT is sulfur poisoning of the LNT in which active sites within the LNT are poisoned by sulfur. Sulfur poisoning occurs when the engine is operated with fuel containing sulfur and an accumulation of the sulfur contaminant builds up in the LNT and causes a decrease in the amount of NOx the LNT can absorb. In order to remove the sulfur contaminant the LNT may be regenerated in what is known as a DeSOx purge regeneration (desulphation). In a DeSOx purge regeneration the temperature of the LNT is increased to circa 675° C. and the sulfur contaminant is burned off. One known method for raising the temperature of the LNT from its usual operating temperature to the temperature required for purging it of sulfur employs the combination of the hot DPF regeneration mode in which no feedgas EGR NOx control is employed and the rich calibration used also for the DeNOx purge. However, this approach to SOx purging regeneration increases NOx emissions during the hot lean phases.
According to a first aspect of the disclosure there is provided a method of operating a multi-cylinder lean burn engine arranged to supply exhaust gas to a lean NOx trap. The method includes checking whether regeneration of the lean NOx trap is indicated and whether a current temperature of one of the lean NOx trap and exhaust gas supplied to the lean NOx trap is above a threshold temperature to permit efficient regeneration of the lean NOx trap. When regeneration of the lean NOx trap is indicated and the current temperature of one of the lean NOx trap and the exhaust gas supplied to the lean NOx trap is not above the threshold, the method includes operating the engine in a lean NOx trap heating mode in which at least one cylinder of the engine is operated rich of stoichiometric in order to increase the temperature of the lean NOx trap and at the same time operating at least one of the remaining cylinders of the engine lean of stoichiometric. The method further includes, when the temperature of one of the lean NOx trap and the exhaust gas supplied to the lean NOx trap is above the threshold temperature, regenerating the lean NOx trap. During the lean NOx trap heating mode, the number of cylinders of the engine operated rich and the respective air-fuel ratio of the mixture supplied to the rich cylinders of the engine and the number of cylinders of the engine operated lean and the respective air-fuel ratio of the mixture supplied to the lean cylinders of the engine are set so as to produce the demanded air-fuel ratio of the exhaust gas flowing to the lean NOx trap and to meet a current torque demand for the engine and the at least one cylinder of the engine that is operated rich is changed in a sequential manner so that all of the cylinders of the engine are operated rich at some time during the period of time in which the engine is operated in the lean NOx trap heating mode.
The engine may have more than two cylinders, more than one cylinder of the engine may be operated rich, and more than one air-fuel ratio may be used for the cylinders of the engine operating rich. The engine may have more than two cylinders, more than one cylinder of the engine may be operated lean, and more than one air-fuel ratio may be used for the cylinders of the engine operating lean. Operating the engine in the lean NOx trap heating mode may result in an air-fuel lambda ratio of the exhaust gas flowing to the lean NOx trap that is not less than one.
Regenerating the lean NOx trap may comprise heating the lean NOx trap to a temperature high enough to permit efficient NOx purge regeneration of the lean NOx trap (e.g., to the threshold temperature or higher) and supplying exhaust gas to the lean NOx trap having an air-fuel ratio less than one. Alternatively, regenerating the lean NOx trap may comprise heating the lean NOx trap to a temperature high enough to permit efficient DeSOx purge regeneration of the lean NOx trap and switching the air-fuel lambda ratio of the exhaust gas flowing to the lean NOx trap between more than one and less than one in a cyclic manner during the DeSOx purge regeneration.
In another example, a method includes, responsive to an indication to regenerate a lean NOx trap (LNT), operating an engine with an overall rich air-fuel ratio to regenerate the LNT while minimizing fuel oil dilution by operating each cylinder of the engine with an alternating rich to lean air-fuel ratio pattern of two rich combustion events for every one lean combustion event across a plurality of engine cycles.
In this way, during the regeneration of the LNT, a similar pattern of alternating rich and lean combustion events may be employed to lower the air-fuel ratio, as needed to carry out the regeneration. However, by interrupting the rich combustion events with one or more lean combustion events, the amount of fuel that can accumulate on the cylinder walls may be reduced, thus lowering the dilution of the engine oil supply with fuel, as may frequently occur during continuous periods of rich operation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.